This invention relates to sand control in oil and/or gas wells by means of sand consolidating plastic and more particularly to the detection of the placement of the plastic.
Oil and gas are produced from underground formations through wellbores drilled down from the surface to penetrate the formation. Some hydrocarbons are contained in the pores of consolidated rocks. When these hydrocarbons (oil, gas, or both) are produced through the wellbore, they are generally free of sand. However, many wells produce fluid from hydrocarbon-bearing formations which are incompetent or unconsolidated; that is, the sand particles are not strongly attached to each other, and, when the fluid (possibly also with substantial amounts of water) is produced, it tends to carry entrained sand with it. This sand can cause extremely serious damage to well equipment. In producing from such unconsolidated formations, some method should be provided to restrain the sand flow into the well.
Generally a sand control operation is performed by packing off the zone of interest and injecting plastic into the formations surrounding the well and setting the plastic. This can be done either by injecting plastic-coated sand particles, which are consolidated when the plastic is set (the injection of coated particles is described in U.S. Pat. Nos. 3,123,137, issued to Young, et al., on Mar. 3, 1964; see also U.S. Pat. Nos. 2,815,815, issued to Hower, et al. on Dec. 10, 1957), or by injecting a plastic which is then set to consolidate the formation sand.
With either method, however, there has been no practical way to control the flow to inject enough plastic in all of the wellbore openings to consistently prevent sand production. The plastic is relatively expensive and thus it is not economically feasible to inject extremely large quantities of the plastic. In addition, some degree of porosity must be maintained to allow production of oil and/or gas and a large quantity injection of plastic generally would result in blocking of the flow. The injected plastic predominantly flows into those areas where there is little resistance to flow. In other areas (such as those with some blockage by a drilling mud or such as an area with much lower permeability, for example) little or no plastic is injected. Such inadequate placement of the plastic will result in high sand flow when the well is put on production. In such cases, the well must generally be shut in, the sand cleaned out of the wellbore, piping, and equipment, and additional sand consolidation performed. This is extremely timeconsuming and expensive.
Various types of plastics have been used for sand consolidation. Phenol-formaldehyde, epoxy resins, and furfuryl alcohol resins have all been used in sand consolidation, as described in, for example, U.S. Pat. Nos. 2,378,817, issued to Wrightsman, et al., on June 19, 1945; U.S. Pat. No. 2,476,015, issued to Wrightsman on July 12, 1949; U.S. Pat. No. 3,100,527, issued to Hilton, et al., on Aug. 13, 1963; U.S. Pat. No. 3,176,768, issued to Brandt, et al., on Apr. 6, 1965; U.S. Pat. No. 3,199,590, issued to Young on Aug. 10, 1965; and U.S. Pat. No. 3,209,826, issued to Young on Oct. 5, 1965. Consolidation with polyurethane is described in U.S. Pat. No. 3,189,091, issued to Bearden and Spurlock on June 15, 1965.
Pulsed neutron logs with tracers in the injected liquids have been used in cased wells for various purposes (but apparently not for the evaluation of sand consolidation). U.S. Pat. No. 3,631,245, issued to Jorden on Dec. 28, 1971; U.S. Pat. No. 3,825,752, issued to Murphy and Froning on July 23, 1974; and U.S. Pat. No. 3,852,593, issued to Robinson on Dec. 3, 1974, illustrate the technique of determining the residual oil saturation by pulsed neutron measurements. U.S. Pat. No. 3,480,079, issued to Guinn, et al., on Nov. 25, 1969, illustrates the use of a different logging technique with analysis to plan the next step in connection with hydraulic fracturing.